Development of a Tc-99m labeled sigma-2 receptor-specific ligand as a potential breast tumor imaging agent

A novel in vivo imaging agent, Tc-99m labeled [(N-[2-((3'-N'-propyl-[3,3.1] aza-bicyclononan-3 alpha -yl)(2"-methoxy-5-methyl-phenylcarbamate)(2-mercap toethyl)amino)acetyl]-2-aminoethanethiolato] technetium(V) oxide), [Tc-99m] 2, displaying specific binding towards sigma-2 receptors was prepared and c haracterized. In vitro binding assays showed that the rhenium, surrogate of [Tc-99m]2, Re-2, displayed excellent binding affinity and selectivity towa rds sigma-2 receptors (K-i = 2,723 and 22 nM for sigma-1 and sigma-2 recept or, respectively). Preparation of [Tc-99m]2 was achieved by heating the S-p rotected starting material, 1, in the presence of acid, reducing agent (sta nnous glucoheptonate) and sodium [Tc-99m]pertechnetate. The lipophilic race mic mixture was successfully prepared in 10 to 50% yield and the radiochemi cal purity was >98%. Separation of the isomers, peak A and peak B, was succ essfully achieved by using a chiralpak AD column eluted with an isocratic s olvent (n-hexane/isopropanol; 3: 1; v/v). The peak A and peak B appear to c o-elute with the isomers of the surrogate, Re-2, under the same HPLC condit ion. Biodistribution studies in tumor bearing mice (mouse mammary adenocarc inoma, cell line 66, which is known to over-express sigma-2 receptors) show ed that the racemic [Tc-99m]2 localized in the tumor. Uptake in the tumor w as 2.11, 1.30 and 1.11 %dose/gram at 1, 4 and 8 hr post iv injection. respe ctively, suggesting good uptake and retention in the tumor cells. The tumor uptake was significantly, but incompletely, blocked (about 25-30% blockage ) by co-injection of "cold" (+)pentazocine or haloperidol (1 mg/Kg). A majo rity of the radioactivity localized in the tumor tissue was extractable (>6 0%), and the HPLC analysis showed that it is the original compound, racemic [Tc-99m]2 (>98% pure). The distribution of the purified peak A and peak B was determined in the same tumor bearing mice at 4 hr post iv injection. Th e tumor uptake was similar for both isomers, but the blood and peripheral t issue content for the isomer in peak B was higher than that for the isomer in peak A. It is evident that the isomer in peak A displayed significantly better tumor/blood and tumor/muscle ratios. The higher rate of in vivo meta bolism was also confirmed by the higher thyroid uptake values for the isome r in peak B as compared to peak A. in summary, a Tc-99m-labeled sigma recep tor imaging agent. [Tc-99m]2, has demonstrated the feasibility of using a T c-99m-labeled agent for imaging sigma receptor expression in tumor cells. T his is the first time a subtype-selective Tc-99m-labeled agent for imaging sigma receptor sites is reported. (C) 2001 Elsevier Science Inc. All rights reserved.